Leaky wave guide planar array antenna

ABSTRACT

A planar array antenna for producing four squinted beams required for an airborne Doppler navigation system is disclosed. The antenna includes a pair of slotted feed rectangular wave guides arranged to permit input energy to be applied at any one of four ports. Interconnecting and coupled to the feed wave guides by means of slots in the feed wave guides is a radiating member which includes a leaky grid structure through which radiates beam-forming electromagnetic energy. The antenna is a symmetrical structure and the four beam positions, or for the case of a lobe switched antenna system, the four beam sets, are symmetrically located with respect to the vertical axis of symmetry of the array.

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Schwartz et al..

[ 1March 20, 1973 LEAKY WAVE GUIDE PLANAR ARRAY ANTENNA OTHER PUBLlCATIONS IRE Transactions October, 1959, pp. 320-329 A Flush-Mounted Leaky-Wave Antenna with Predictable Patterns, by Honey.

Primary Examiner-John S. Heyman [73] Assignee: 51h; Singer Company, Little Falls, Acimatana et [22] Filed: Aug. 16, 1971 [57] ABSTRACT [21] Appl. No.: 171,900 A planar array antenna for producing four squinted beams required for an airborne Doppler navigation system is disclosed. The antenna includes a pair of [52] US. Cl. ..343/756, 343/768, 343/771 Slotted feed rectangular wave guides arranged to pen [51] 11112. Cl. ..H0lg 13/00 mit input energy to be applied at any one of four [5 8] Field of Search ..343/767, 768, 769, 770, 771, interconnecting and coupled to the feed wa've 343/854 897 guides by means of slots in the feed wave guides is a radiating member which includes a leaky grid struc- References ture through which radiates beam-forming electromagnetic energy. The antenna is a symmetrical UNITED STATES PATENTS structure and the four beam positions, or for the case 3,083,362 3/1963 Stavis ..343/771 of a lobe switched antenna system, the four beam sets, 3,137,856 6/1964 Tashjian... ..343/854 are symmetrically located with respect to the vertical 3,281,851 10/1966 Goebels .343/768 axis of symmetry of the array 3,508,275 4/1970 Deveau et al.... ..343/77l X 3,599,216 8/1971 Paine ..343/771 11 Claims, 7 Drawing Figures x x v Y I l2 X I :1 x r X PATENTfinnARzo 1915 SHEET 10F 3 INVENTORS LEONARD SCHWARTZ DONALD Z. B LAU 8| EDWARD CHIN %M3/ gvv fia y /S\' -LMW ATTORNEYS PATETNIEDHTRoma SHEET 3 BF 3 k FIE H livh :y g w FIG.7.

CONSTANT Y INVENTORQ LEONARD SCHWARTZ DONALD Z. BLAU 8 EDWARD cHm /ATTORNEYS LEAKY WAVE GUIDE PLANAR ARIRAY ANTENNA BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to antennas and more particularly to a planar array of elements for radiating high frequency electromagnetic energy so that the energy is confined to form a highly directional beam.

2. Description of the Prior Art A well-known antenna structure for Doppler navigation systems uses slotted wave guide arrays in linear or planar configurations to achieve the required beam geometry. The radiating wave guides include normally resonant slots which are positioned along the wave guide to couple power out at discrete locations. An exemplary slotted wave guide planar array is disclosed in U.S. Pat. No. 3,276,026. The antenna disclosed in said patent includes a pair of slotted feed wave guides arranged to permit input energy to be applied at any one of four ports. Interconnecting and coupled to the feed wave guides is a plurality of radiating wave guides, each having resonant slots to couple power out. While slotted wave guide planar array antennas are suitable in many situations, they entail certain problems in their manufacture and use.

A significant problem encountered with slotted wave guide antennas is that expensive machining and assembly methods must be used. According to Table [ll of the aforementioned patent, each of the radiating slots have different angulation and depth to provide for resonant coupling. Also, assembly of the planar array according to the aforementioned patent requires that mechanical connections be made between each radiating wave guide. This leads to a considerable amount of assembly time. Thus, it should be evident that antennas according to the aforementioned patent pose significant manufacturing cost obstacles which are not capable of being readily overcome by mass production techniques.

Another significant and longfelt problem encountered with slotted wave guide antennas in known Doppler navigation systems is over water calibration shifts. These are caused by the fact that when microwave radiation is reflected from a water surface the amount of energy returned to the receiver-transmitter depends not only on the distance but also on the angle of incidence of the radiation at the water surface; being much reduced at acute angles to the horizontal direction. Various techniques are known for reducing over water calibration shifts with slotted wave guide antennas. One technique uses lobe switching which requires rather involved microwave systems, as described in U.S. Pat. No. 3,113,308. A more simplified technique is the use of beam shaping as described in the U.S. Pat. No.- 2,983,920 wherein the beam shaping involves a pattern contour which results in an invariant Doppler spectrum central frequency over water. Although it would be very desirable to employ beam shaping, it has not been practically employed in current Doppler navigation systems primarily because efficient economic antenna configurations have not existed.

In contrast to the discrete coupling from slotted antennas discussed above, an alternate concept of coupling and construction has been known since at least the early 1940's. This alternate involves continuous leakage of energy along the length of a so-called leaky wave guide." Some of the rudimentary operational parameters of leaky wave guides are described in U.S. Pat. No. 2,405,242 which was granted in 1946. A decade later, the leaky wave guide was understood more thoroughly by workers in the art and theoretical and experimental antenna design data began to be published in the literature. Exemplary of such literature is a paper by RC. Honey entitled A Flush-Mounted Leaky-Wave Antenna with Predictable Patterns published in October 1959 in LRE. Transactions on Antennas and Propagation. That paper describes a leaky wave antenna consisting of an inductive sheet spaced over a continuous conducting sheet with a wave polarized in the plane of the antenna propagating between the two sheets and providing radiated energy through the inductive sheet to form a beam. Honey's paper indicates that few, if any, leaky wave antennas had found practical use prior to his work, principally because such antennas require that the phase velocity and the rate of leakage along the antenna must be known very accurately in order to design large antennas with prescribed radiation patterns. The paper alleviates some of these design difficulties by reporting an analytical method of predicting the radiation properties of a leaky wave antenna and by comparing them with the results obtained experimentally.

SUMMARY OF THE INVENTION An object of the present invention is to provide an improved antenna for airborne Doppler navigation systems.

Another object of the present invention is to provide a Doppler antenna which can be readily manufactured, particularly an antenna in which relatively expensive prior art machinery and assembly methods are eliminated.

Still a further object of the invention is to provide a Doppler antenna wherein a radome is provided as a matter of course when the improved manufacturing methods are employed.

A further object of the invention is to provide a Doppler antenna which will allow more power to be radiated from the radiating wave guide than from the prior art antennas.

Other objects and advantages of the present invention will be apparent to those skilled in the art from the description of the preferred embodiments of the invention which follows.

Applicants have discovered that the above objects are accomplished by an antenna which includes a pair of slotted feed wave guides arranged to permit input energy to be applied at any one of four ports. Interconnecting and coupled to the feed wave guides by means of slots in the feed wave guides is a radiating wave guide which includes a leaky grid structure through which radiates electromagnetic energy. The leaky grid is formed from a metal clad insulating material which has been photoetched to form a pattern of parallel conducting strips. When assembled, the dielectric material forms the radome. The applicants have discovered that the antennaaccording to the present invention exhibits unexpectedly superior performance in alleviating the longfelt problem of over water calibration shifts. Thus, the antenna of the present invention provides the synergistic factors of reduced cost, increased ease of manufacture and improved performance to alleviate a longfelt problem in the art.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a planar array and the beams radiated thereby for Doppler systems.

FIG. 2 is an orthographic view ofthe array showing DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a planar antenna array which customarily is mounted horizontally in an aircraft so that when the aircraft is in straight and level flight, the radiated beams are directed downwardly toward the earth. The array is symmetrical with respect to orthogonal axes X, Y and Z. A pencil beam, which may assume any of the positions indicated by the beams 11., 12, 13 or 14, is radiated from the array dependent upon which one of four input ports of the array is energized, as will be more fully discussed below. The term pencil beam refers to a highly directive antenna pattern consisting of a single major lobe contained within a cone of small solid angle and almost circularly symmetrical about the direction of peak intensity. It is also possible by using a dual beam feed array to obtain two major lobes for each input port. This type of antenna pattern is required in a lobeswitch Doppler navigation system. The beams l1, l2 and 13 and 14 are located at an angle 1 with respect to the vertical axis I, and 'y with respect to the horizontal X-axis and at an angle awith respect to the Y-axis or the transverse axis of the aircraft.

Referring now to FIG. 2, the planar array 10 is shown in greater detail. A pair of feed wave guides 21 and 22 are shown having a pair of input ports 31-32 and 33-34, respectively. The ports are adapted to be connected to a source of energy (not shown). The input ports can be either adapted to the energy source as shown or alternately adapted to the energy source through a hole cut in the metallic ground plane inside the aperture of the radiating device. Disposed between the feed wave guides 21 and 22 and coupled thereto is a radiating wave guide 41 which includes a leaky grid 42 which consists of metal strips photoetched on one side of a dielectric plate and which is an integral part of the antenna and which serves as the radome, a contoured metal ground plate 43 and side walls 44. Microwave absorbers 46 may be positioned along the inside of the side walls 44 for selective reduction of the intensity of undesirable reflections. The shape and extent of the microwave absorber can be selectively chosen to effect an optimum absorption of microwave reflections within the leaky wave guide. The leaky grid 42 is a dielectric plate with a grid of flat parallel metal strips 45 of desired spacing, width and thickness on the inside wall. The spacing of the strips 45 determines the proportion of the energy in the waveguide which is radiated to form beams 11, 12, 13 and 14. Either dielectric or metallic spacers 61 can be placed inside the leaky wave guide to provide support and maintain tolerance for the required plate separation. Furthermore, the spacers will provide mechanical integrity to the overall design. The construction and design of the leaky grid and ground plate, etc. will be discussed further below.

Referring now to FIG. 3, there is shown the junction of the feed wave guide 22 and the radiating wave guide 41. The feed wave guide 22 is shown having a plurality of slots 25 spaced by a distance d. These slots couple energy from the feed to the radiating wave guide. The

construction and design of the feed wave guide will be discussed further below.

In order to achieve four symmetrically located pencil beams shown in FIG. 1, the amplitude function along the feed and leaky radiating wave guide structure are chosen to be asymmetrical around the center line of the array, that is, a function is selected along one half of the array when fed from one end (e.g. from port 33 or 34) and this function is repeated asymmetrically so that it is obtained on the other half of the antenna when fed from the other end (e.g. from port 31 or 32). Thus, the radiation pattern will be the same regardless of which end of the antenna is excited.

Unlike the wave guide planar array of the aforesaid U.S. Pat. No. 3,276,026 where each feed slot is coupled to a radiating array, the feed array in the present invention radiates directly into the parallel plate region of the radiating wave guide 41. Thus, as shown in FIG. 4, the wave front propagates along the parallel plate wave guide with the direction imparted by the feed wave guide. Excitation at input port 31 will result in a propagation direction along the parallel plate wave guide defined by line 31' in FIG. 4, and an excitation at input port 32 resultsin line 32'. Typical feed angles K required for Doppler antenna systems are 46 and 52 from the feed normal. While the pattern formations for the aforesaid U.S. Pat. No. 3,276,026 may be represented as the product of two orthogonal linear array functions for the feed and radiating arrays, the pattern formations for the present invention are a more complicated function of the two patterns.

The radiating wave guide of FIG. 2 includes a leaky grid structure forming wave guide whose narrow dimension is made much larger than normal. The wide dimension is nominally the same as the wave guide used in the aforementioned U.S. Pat. No. 3,276,026. Using an edge out slotted wave guide feed, it is possible to excite the TE mode along the length of the oversized wave guide. The leaky wave guide structure enables the rate of radiated energy to be controlled to achieve a low side lobe antenna. In addition, there is no restriction on the feed so that a low side lobe pattern in both the feed and radiating direction is obtainable.

In planar arrays, the total radiation pattern may be represented as the product of two orthogonal linear array functions for the feed and radiating arrays. As

mentioned earlier the present invention has a total radiation pattern which is slightly more complicated but nonetheless can be represented approximately as a product of two functions; one which is dependent upon the radiating leaky grid structure. Unlike the pattern formations for the aforesaid US. Pat. No. 3,276,026, the present invention has a feed pattern which is dependent on the radiating leaky grid structure as will be explained in greater detail below.

The tilt in the wave front shown in FIG. 4 as the angle a, is obtained by using either an inpha se or antiphase edge out slot array. For the case of an inphase edge cut slot array,

cosa=)tp/)\gi \p(2N)/2d (1) where )tp is the wavelength inside the radiating leaky grid structure which can be considered as a parallel plate region, N is an integer which determined the order of the lobes, d is the slot spacing, and Ag is the wavelength inside the rectangular feed wave guide. The antenna shown in FIG. 1 radiating four pencil beams is one in which N 0, or one in which the feed wave guide only radiates one principal lobe per input port excitation.

For the case of an antiphase edge out slot array,

cosa=)\p/ \g(N+ /2))tp/2d (2) where again the design can include the generation of higher order lobes.

The design of the radiating leaky grid and contour combination is such as to maintain a constant )tp value throughout the radiating wave guide portion of the antenna. Thus, a well focused beam appears at an angle which is the complement of I in FIG. 1 and is determined by where A is the operating wavelength. The squint in the feed direction or central 7 angle, as shown in FIG. 1, is

completely determined since cos7=cosasinl'. (4

Also, the squint in the radiating wave guide direction or central 0' angle, as shown in FIG. 1 is determined by sinv=sin1l1sina 5 where v 90 0'.

For a rectangular wave guide propagating the dominant or TE mode, the wavelength Ag is given by Ag= l- [A/(2a,,)] (6) where a is the wide dimension of the wave guide. In contrast to this simple expression, the wavelength hp of the leaky wave guide or parallel plate region is given by a complex function where b is the strip width of the leaky wave guide antenna, s is the center-to-center spacing between strips as shown in FIG. 3, and A is the height of the parallel plate region from the contoured reflector to the strips as shown in FIG. 3.

The significance of equation 7 is that as the s dimension is varied to obtain the desired leakage rate, the

A,,"dimension must be simultaneously varied to keep Ap constant. This is necessary in order to keep the peak beam angle constant along the array. Thus, the ground plate 43 must be shaped to provide the desired A,, dimensions along the radiating wave guide. FIG. 5 illustrates such a shaped metal ground plate apart from the rest of the antenna. In FIG. 5 the E vector is parallel to the feed wave guides. For a more detailed analysis of the mathematical model for the leaky wave guide, the aforementioned paper by I-Ioney should be consulted. Applicants intend that the subject matter of that paper be incorporated herein. As mentioned above, applicants have discovered that the antenna of the present invention exhibits unexpectedly superior over water performance due to the beam contour produced. One explanation for the desirable beam contours obtained for the leaky grid antenna is related to the phase front direction from the feed being nonparallel to the metal grid as discussed above with reference to FIG. 4. The design of the grid assumes a wave traveling normal to the grid as is indicated by line A in FIG. 4. Due to the need to generate four symmetrical beams from a common aperture, the feed wave front appears alternately on either side of the center line as shown in FIG. 4. Thus, designing the grid based on a wave traveling normal to the grid is necessary for the four beam antennas. The wave guide wavelength Mtp along the grid is kept constant along the direction parallel to the feed by varying the ground plate as discussed above. Experimental data for the leaky grid patterns show a trend of a wider beam width and a smaller angle from the broadside direction as the feed propagation angle is increased from the normal.

Since the feed propagation angle is approximately 45 from the normal, there exists, in effect, a partial slanted array. The wave front propagated sees a slanted array at least until one of the side walls is encountered. It is a known fact in the art that a purely slanted array can eliminate the Doppler over water calibration shift by generating a beam whose power density contour diagram is shaped as an ellipse with prescribed ellipticity. The Applicants believed that this explanation accounts not only for the wider measured beam width in one direction but also for the improved over water performance of the antenna. One consequence of the above explanation is that a greater amount of energy reflection off the side wall will result since the wave front propagation not only sees a slanted array, it also sees less area to radiate the electromagnetic energy. This effect does not significantly reduce the ability of the antenna since the construction of the antenna is such as to allow microwave absorbing material to be placed along the side walls to reduce undesired reflections.

FIG. 6 shows typical 3 and -5 db beam contours of a single measured beam. These contours are close to the beam shaping antennas discussed in the aforesaid US. Pat. No. 2,983,920. Applicants have found over water improvements of up to an order of magnitude better than comparable wave guide planar arrays.

FIG. 7 illustrates an additional grid construction which may be used to reduce undesired cross-polarized side lobes. Experimentally, applicants determined that the cross-polarized energy was emitted primarily in the region adjacent to the slotted wave guide feeds. By

using a grid of wires or strips 60 orthogonal to the radiating grip strips 45 in the vicinity of the feed laucher 22, it is possible to substantially reduce (8 to ID db in the angular region of interest) the cross-polarized energy. The cross grid acts as a reflector to the crosspolarized energy while allowing the normal polarization to leak out with minor attenuation. This grid may be combined with the original radiating grid and photoetched in a single processing step.

As an example of the present invention, a leaky wave guide planar array antenna was fabricated incorporating a leaky grid and conducting ground plane. The leaky grid structure was fabricated with a photoetched grid of flat strips on a fiber glass skin and sandwiched with a hexcell fiber glass cell and an additional fiber glass skin to form a composite grid-radome structure. The technique of the radome fabrication resulting in a half-wavelength A" sandwich type radome is well known in the state of the art. The conducting ground plane was fabricated as a honey comb structure wherein one skin of the honey comb structure had material thickness sufficient for making a required onedimensional contour. The required separation A,," between the contoured side of the metallic ground plane and the photoetched grid contained on the dielectric radome member was provided by side wall metallic bars. In addition, the plate flatness was also controlled by the side wall bars. The radiating aperture size of the antenna was 13 inches by 14 inches. Feedlaunch mechanisms were located orthogonal to the prescribed ground place one dimensional contour at both ends of the leaky wave guide array. Rectangular slotted feed wave guides were located at both ends of the leaky planar array and collared in position by the use of dielectric L shaped clamps epoxied to both the conducting ground plate and rectangular feed wave guides.

The printed grid was oriented so that the grid of flat copper strips faced the ground conducting plate. The copper laminate comprising the grid was 0.0014 inches thick. The 0.0014 inches of copper exceeded the depth of conduction current or skin depth of copper by ap proximately 15 times.

The nominal A, dimension of the antenna was controlled in the aperture area of the antenna by employing dielectric spacers of appropriate shape and tolerance dimensions. Applicants have found that the placement of either dielectric or metallic cylinders in the aperture area causes only small changes in the measured beam radiation patterns. in the antenna, the feedlaunch cover plate is a part of the copper-laminate fiber glass and is photoetched just as the grid.

The following procedures were followed in fabricating the flat strip grid. The grid strip requirements were d 0.030", s (variable in order to maintain a constant kp), and N 122. In order to photoetch the copper fiberglass laminate, a scaled photographic master was required. The initial ruled master before reduction, measured 23.00" X 24.620". The aperture area before reduction measured 18.760" X 20.010". Included on the photographic master were peripherial regions which would act as launch cover plates and side wall electrical contact bars.

The ruled master was then photographically reduced by a factor of 0.l39/0.20O 0.695 or 60.5 percent.

This photographic master was then used to photoetch the copper laminate fiber glass skin.

As various possible modifications might be made in the embodiments set forth, it is to be understood that all the matters herein described or shown in the accompanying drawings are to be interpreted as illustrative and not limitative to exact details.

We claim:

1. A multiple beam planar array antenna, comprising wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a plurality of parallel metal strips and a second wall having varied spacing from said first wall along the length of said radiating wave guide; wave guide means for feeding energy to said radiating wave guide including two elongated feeder wave guides, said feeder wave guides having a plurality of slots, respectively, communicating with said radiating wave guide to couple energy from said feeder wave guides to said radiating wave guide each of said two feeder wave guides having on each end an input port for coupling said antenna to a source of energy whereby by selectively supplying energy to said input ports the energy will leaked from said elongated wave guide in such a manner as to form said multiple beams.

2. The invention of claim 1 wherein energy absorbing means are mounted in said radiating wave guide for reducing the intensity of undesirable reflections within the structure in order to improve patterns.

3. The invention of claim 1 wherein said slots are arranged in an inphase array.

4. The invention of claim 1 wherein said slots are arranged in an antiphase array.

5. The invention of claim 1 wherein one wall of each of said two elongated feeder wave guides is formed by a strip of metal photoetched on said laminate.

6. The invention of claim 4 wherein a plurality of metal strips are arranged orthogonally with said parallel metal strips for reducing cross-polarization.

7. The invention of claim 1 wherein said parallel metal strips are selectively spaced to provide desired beam shapes and directions to correct for over water calibration shift for Doppler navigation systems.

8. The invention of claim 3 wherein each of said input ports is designed and constructed to provide two major beams, the purpose of said two beams being to facilitate a lobed-switch Doppler system.

9. The invention of claim 1 wherein dielectric or metallic spacers are located within the radiating wave guide means to provide mechanical support for said walls.

10. A multiple beam planar array antenna, compris ing wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a plurality of parallel metal strips formed of sheets of photo etched laminate of metal and dielectric, the dielectric sheet forming a radome for said antenna and a second wall having varied spacing from said first wall along the length of said radiating wave guide; wave guide means for feeding energy to said radiating wave guide including two elongated feeder wave guides, said feeder wave guides having a plurality of slots, respectively, communicating with said radiating wave guide to couple energy from said feeder wave guides to said radiating wave guide.

11. A multiple beam planar array antenna, comprising wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a first plurality of paralled metal strips and a second plu- 10 

1. A multiple beam planar array antenna, comprising wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a plurality of parallel metal strips and a second wall having varied spacing from said first wall along the length of said radiating wave guide; wave guide means for feeding energy to said radiating wave guide including two elongated feeder wave guides, said feeder wave guides having a plurality of slots, respectively, communicating with said radiating wave guide to couple energy from said feeder wave guides to said radiating wave guide each of said two feeder wave guides having on each end an input port for coupling said antenna to a source of energy whereby by selectively supplying energy to said input ports the energy will leaked from said elongated wave guide in such a manner as to form said multiple beams.
 2. The invention of claim 1 wherein energy absorbing means are mounted in said radiating wave guide for reducing the intensity of undesirable reflections within the structure in order to improve patterns.
 3. The invention of claim 1 wherein said slots are arranged in an inphase array.
 4. The invention of claim 1 wherein said slots are arranged in an antiphase array.
 5. The invention of claim 1 wherein one wall of each of said two elongated feeder wave guides is formed by a strip of metal photoetched on said laminate.
 6. The invention of claim 4 wherein a plurality of metal strips are arranged orthogonally with said parallel metal strips for reducing cross-polarization.
 7. The invention of claim 1 wherein said parallel metal strips are selectively spaced to provide desired beam shapes and directions to correct for over water calibration shift for Doppler navigation systems.
 8. The invention of claim 3 wherein each of said input ports is designed and constructed to provide two major beams, the purpose of said two beams being to facilitate a lobed-switch Doppler system.
 9. The invention of claim 1 wherein dielectric or metallic spacers are located within the radiating wave guide means to provide mechanical support for said walls.
 10. A multiple beam planar array antenna, comprising wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a plurality of parallel metal strips formed of sheets of photo etched lamiNate of metal and dielectric, the dielectric sheet forming a radome for said antenna and a second wall having varied spacing from said first wall along the length of said radiating wave guide; wave guide means for feeding energy to said radiating wave guide including two elongated feeder wave guides, said feeder wave guides having a plurality of slots, respectively, communicating with said radiating wave guide to couple energy from said feeder wave guides to said radiating wave guide.
 11. A multiple beam planar array antenna, comprising wave guide means for radiating said beams including an elongated radiating wave guide for leaking energy continuously along its length to form said beams, said radiating wave guide including a first wall having a first plurality of paralled metal strips and a second plurality of metal strips arranged orthagonally thereto for reducing cross polarization and a second wall having varied spacing from said first wall along the length of said radiating wave guide; wave guide means for feeding energy to said radiating wave guide including two elongated feeder wave guides, said feeder wave guides having a plurality of slots, respectively, communicating with said radiating wave guide to couple energy from said feeder wave guides to said radiating wave guide. 